1. Field of the Invention
The present invention relates to a fixing device for fixing a toner image formed on a recording sheet, onto the recording sheet, as well as to an image forming apparatus having the fixing device.
2. Description of the Related Art
An electrophotographic image forming apparatus for forming images by means of electrophotography, such as a copying machine and a printer, is provided with a fixing device. As the fixing device, as shown in FIG. 2 for explaining the invention which will hereafter be described, there is known a fixing device 7 having a heat roller 71 which is heated by a heating portion and a pressure roller 72 which is brought into contact with the heat roller 71 under a predetermined contact pressure. In the fixing device 7, a recording sheet, for example, recording paper 202, having a yet-to-be heated developer e.g. toner transferred to its surface 202a which served as an image surface in a transfer device at a preceding step and is now going to face the heat roller 71, is fed between the heat roller 71 and the pressure roller 72 so that the yet-to-be heated developer is fixed onto the recording sheet 202.
In order to convey the recording sheet 202, the fixing device 7 has a post-fixing roller 75 and a post-fixing driven roller element 76 which rotates depending on the rotation of the post-fixing roller 75, that are located posteriorly of the heat roller 71 in a direction P in which the recording sheet 202 is conveyed; that is, located downstream of the heat roller 71 in a conveyance direction P. As the post-fixing roller 75 is rotated, with the recording medium 202 caught in the space between the post-fixing roller 75 and the post-fixing driven roller element 76, these rollers convey the recording sheet 202.
FIG. 13 is an exploded perspective view showing a heat roller 300 in a disassembled state, which is provided in a fixing device 400 according to a related art, and FIG. 14 is a side view of the heat roller 300, as viewed from one side in an axial direction thereof. In FIG. 14, a one-side rotary shaft 302 is illustrated by cross-hatching. The one-side rotary shaft 302 is a rotary shaft which extends in one axial direction from a roller main body 301 of the heat roller 300. In the heat roller 300, a key 311 formed in a fixing gear 310 fitted to a keyway 303 formed in the one-side rotary shaft 302, whereby the fixing gear 310 and the roller main body 301 are coupled to each other and are thus allowed to rotate together. The keyway 303 is so formed as to prevent the key 311 from moving along a direction in which the heat roller 300 is rotated. To be specific, the keyway 303 extends linearly in the axial direction and is substantially the same in size as the key 311 when viewed along the rotation direction of the heat roller 300.
FIG. 15 is a view for explaining a transmission mechanism of a driving force toward the heat roller 300 and the post-fixing roller in accordance with the related art. The heat roller 300 and the post-fixing roller are driven to rotate by a driving section 321. In order to rotate the heat roller 300 and the post-fixing roller, the driving section 321 uses a motor (not shown) to rotate a paper discharge roller-side gear 320 which is engaging with the fixing gear 310, thus causing the fixing gear 310 to rotate. The driving force exerted by the motor is transmitted in a direction indicated by an arrow B from the paper discharge roller-side gear 320 to the fixing gear 310 through which the driving force is transmitted in a direction indicated by an arrow C to the heat roller 300. The post-fixing roller is rotated under the driving force transmitted from the fixing gear 310, through a first coupling gear 322 and a second coupling gear 323, to a post-fixing roller driving gear 324.
As shown in FIG. 2 which will hereafter be explained, the heat roller and the pressure roller are each constructed of an elastic roller. Therefore, in a case where the recording sheet 202 is of the type that has a certain degree of thickness, such as gloss paper or heavy paper, upon passing of the recording sheet 202 through a nip region 203, the heat roller and the pressure roller undergo elastic deformation owing to the recording sheet 202, in consequence whereof there results a buildup of elastic energy in both of the rollers. The elastic energy is released when the recording sheet 202 comes out of the nip region 203, thus causing speeding up of the heat roller and the pressure roller. The resultant accelerated rotation of the heat roller reverses the transmission direction of the driving force exerted by the motor.
FIG. 16 is a view for explaining a mechanism for reversing a direction of power transmission. As shown in FIG. 16, the accelerated heat roller 300 applies a force to the key 311 of the fixing gear 310, which reverses the transmission direction so that the driving force is transmitted in a direction indicated by an arrow D from the one-side rotary shaft 302 of the heat roller 300 to the fixing gear 310. This causes momentary acceleration of the rotating fixing gear 310, with the result that the driving fore is transmitted in a direction indicated by an arrow E from the fixing gear 310 to the paper discharge roller-side gear 320. The paper discharge roller-side gear 320 is driven to rotate by the motor and configured to transmit the driving force in a direction opposite to the direction of the arrow E. The reversing of the transmission direction as above will therefore cause so-called tooth jumping between the fixing gear 310 and the paper discharge-roller side gear 320.
Upon the re-engagement between the fixing gear 310 and the paper discharge roller-side gear 320, the recording sheet is returned to a normally conveyed state where the tooth jumping has not occurred as yet. However, as shown in FIG. 6 for explaining the invention which will hereafter be described, until such time that the fixing gear 310 and the paper discharge roller-side gear 320 engage with each other, the recoding sheet is standing still, with its downstream-side end in the conveyance direction (hereafter referred to as “tail end”) kept at a position to which it is moved immediately after coming out of the nip region 203. During that time, inconveniently, the post-fixing driven roller element 76 kept in contact with the image surface 202a leaves some impressions on the image.
A technique to prevent jumping of gear teeth, although it is not related to fixing device development, is disclosed in Japanese Unexamined Patent Publication JP-A 9-286529 (1997). JP-A 9-286529 discloses a sheet feeding apparatus. In the sheet feeding apparatus disclosed in JP-A 9-286529, when a paper-feeding roller is put under a load which is greater than a predetermined level, the engagement between the paper-feeding roller and a driving shaft is released so that only the driving shaft is rotated, that is, the driving shaft is idled. This makes it possible to reduce the load applied to the driving gear and thereby prevent occurrence of tooth jumping in the driving gear.
If, in the fixing device, the driving shaft is idled to prevent occurrence of tooth jumping in the fixing gear corresponding to the driving gear as in the technique disclosed in JP-A 9-286529, the conveyance of the recording sheet will be brought to a stop. Therefore, just as in the case where the tooth jumping occurs, the post-fixing driven roller element leaves some impressions on the image surface. The technique disclosed in JP-A 9-286529 is thus not applicable to the fixing device.